I have to admit, and it may very well may be me being dense here, but I am not sure you have this correct.
The distributor itself will have so many distributor degrees of advance built in. Different distributors can have different amounts of advance. So for example, a later 235 distributor generally has like 12 distributor degrees of advance built in (24 degrees at the crank). So you are starting out with 24 degrees of total mechanical advance. If you need more advance, say you want 32 of total advance, you then set the distributor at 8 degrees btdc. This is the initial lead and is added to the 24 distributor degrees. You can keep adding initial until the motor pings to get more, or you can modify the internals to get more advance and less initial (a more involved process.)
The next piece to the puzzle is just how fast or slow you want the distributor degrees (the mechanical advance) to come in. This is a function of the weights and springs. Find the specification for the distributor. It should tell you when the timing will start to come in. It's common to see the advance start at like 1500 rpm and have it all in by sometimes as much as 3000 rpm. A lot of cars like it to start in a little earlier, like 1000 or 1200 and have it all in by 2500 rpm. This is accomplished by swapping the heavier springs for some lighter springs. Sometimes one light and one medium, other times both medium or both light. It's trial and error to see what the motor likes, but in any event the total timing is mechanical plus initial. The one issue you will need to be cognizant of is that the light springs wear out and stretch out faster then the heavier springs so if you use them, keep an eye on the timing curve to confirm that this has not happened.
The vacuum advance is only operative at idle and when the motor is running at highway no load speed and is determined by the advance can that is installed (gm had many different part numbers for different vacuum advances) or you engage in some trial and error using an adjustable vacuum advance. But the formula is not initial plus mechanical plus vacuum.
Last edited by mdonohue05; 09/17/17 04:09 PM.
